Orthokeratology or ortho-K (also known as corneal re-shaping or overnight vision correction) is the use of specially designed rigid gas permeable (RGP) contact lenses to alter the shape of the cornea (the transparent front part of the eye) in order to reduce or correct low to moderate levels of myopia (short sight). The procedure may also be effective with low degrees of astigmatism.
Orthokeratology lenses are worn nightly, or on alternate nights, and removed in the morning. The aim is to correct vision without the need for spectacles or contact lenses during the day. Because the effects on the shape of the eye are temporary, the eyes return to their original shape and prescription if the lenses are left off for a few days.
The main advantage is that contact lenses are not worn during the day, which is particularly useful for some activities, such as water or contact sports. Since it's possible to see both with and without the lenses, they correct vision around the clock.
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Specs appeal: the charity bit
If you're really finished with those glasses, drop them in to us, we will carry out some basic repairs and hand them over to a Third World charity. Good deed done!